I don't know where you read that but Rainier was in a fight with Onassis over control of Monaco, especially during the 1960s. Onassis (through his corporation) had bought many of the hotels and casions in Monte Carlo and Fontville. HE was deciding how they were run, who was allowed in and so forth.
Rainier was rather cash-poor (even though he received a few million from Jack Kelly Sr as dowry for Grace) and couldn't immediately buy him and his cronies out. It took several decades and some rather shady money for Rainier to reacquire land in his own country!
Maria Callas was one of Onassis' lovers. Even now, one of the hotel-casinos takes great delight in showing Onassis' former apartment and the secret staircase to Maria's flat above. The suites tour is even in several of the Grimaldi/Princess Grace/Monaco videos that A&E and Biography has done over the years.
Grace was a great patron of music and brought about a resurgence of the Opera Company in Monaco. It was Maria who was friendly with Grace (who was friendly with many performers of different disciplines).
If Rainier and Onassis were in the same concert venue for a performance, charity event or whateve, it was definitely only because their female companions MADE them!
Ann